The Stirling-cycle engine was first conceived and reduced to practice in Scotland 164 years ago. A hot-air closed-cycle prime mover based on the principle was patented by the Reverend Robert Stirling in 1817 as an alternative to the explosively dangerous steam engine. Incredibly, this event occurred early in the Age of Steam, long before the invention of the internal combustion engine and several years before the first formal exposition of the laws of thermodynamics.
Air was the first and only working fluid in early 19th century machines, whereas hydrogen and helium have been the preferred working fluids for modern machines. In Great Britain, Europe, and the United States thousands of regenerative hot air prime movers in a variety of shapes and sizes were widely used throughout the 19th century. The smaller engines were reliable, reasonably efficient for their time, and most important, safe compared with contemporary reciprocating steam engines. The larger engines were less reliable, however, because they tended to overheat and often succumbed unexpectedly to premature material failure.
By 1920, therefore, the electric motor and the internal combustion engine had almost universally and completely replaced the hot air prime mover in the marketplace. Until recently there was little incentive or opportunity to reconsider the commercial exploitation of the Stirling engine's numerous potential advantages. This was partly because the state of the art in many areas of related technology was inadequate.
Since World War II, however, there have been unprecedented advances in the general technologies of machine design, heat transfer, materials science, system analysis and simulation, manufacturing methods, and Stirling engine development.
Today, in comparison to their conventional internal bustion counterparts, all modern Stirling prime movers are external combustion engines which consistently demonstrate, in the laboratory, higher efficiency, multifuel capability, lower exhaust emissions, quieter operation, equivalent power density, and superior torque characteristics. Nevertheless, none of these engines is mass produced for any commercial application anywhere in the modern world. The reason for this is that contemporary Stirling engines have been created largely by adapting traditional methods and designs from the more familiar internal combustion engine technology base, and are therefore complex.
Patchwork adaptation of the old as a shortcut path to the new is a process which inexorably produces a hodge-podge arrangement of excessive mechanical complexity and which inevitably results in high production costs. In the design of innovative high-technology devices, the easy or obvious solution to a design problem at the component level often leads to an unacceptably complex ramification at the system level. Despite clearly superior technical performance characteristics, therefore, contemporary Stirling engines are invariably not cost competitive from the standpoint of economical mass production.